Guto Harri was phoning in his new guise as a Fleishman-Hillard consultant, wanting to know what a Johnson victory would mean for one of the agency's big clients. Johnson took the call, but there was something else he wanted to talk about.

Harri recalls: ‘Before I said anything he asked me whether I'd received a message that he was trying to get hold of me to see if I might be interested in doing something at City Hall if he got elected. It did come slightly out of the blue and I wasn't sure at that point if he was serious or not, so I just said "Let's see if you get elected".'

Johnson was deadly serious. In fact, his advisers had already drawn up a list of big hitters to be his comms chief of which Harri was firmly at the top.

The deal was sealed in Johnson's garden, two days after his election as Mayor. ‘On the Sunday after he was elected, he rang me and suggested we meet for a drink that evening,' reveals Harri. ‘So I went up to Highbury with a good bottle of dry white wine. We sat in the garden and discussed what we might do.'

Eight days later Harri was safely ins­talled in City Hall as Johnson's first comms director. The task ahead is one laden with responsibility. If Johnson performs well, so the argument goes, this will demonstrate to the country that the Conservatives are back in business and that David Cameron's party is ready for government.

If Johnson screws up the capital, then Lab­our has a ready-made slogan for the next election: ‘Don't let Cameron do to Britain what Boris has done to London.'

On one level, Harri's task is to stop the new mayor from making the kind of gaffes that have landed him in trouble in the past. ‘His challenge is to get Boris to say something, but not then go on and say something else,' says one Conservative source.

However, those close to Johnson insist the new mayor is a changed man, and that such media hand-holding is no longer req­uired. ‘Boris understands what is needed now more than before,' says one Johnson aide.

‘Gone are the days when he thought he was an after-dinner speaker and that his role was to entertain people. He now knows that he is there to offer leadership; that people want stability and someone they can trust in the job.'

Katie Perrior, co-founder of InHouse PR, which advised Johnson throughout his campaign to be mayor, says Harri's main role will be to advise on the timing and presentation of key announcements.

‘Guto will be Boris' right-hand man,' says Perrior, whose agency is now launching an offshoot called InHouse London. ‘Boris wants to make his manifesto promises a reality and soon. He's got a jam-packed agenda and Guto is the man who will decide when and where and oversee the whole presentation of it. So Boris will be trusting Guto with his big ideas.'

Harri - who is not overtly political - is said to be well connected in Conservative circles. He and Johnson were rugby-playing contemporaries at Oxford before they both entered political journalism, alt­hough Harri says they were never best mates: ‘I did meet him at Oxford but I wasn't part of his set or anything.'

Harri went on to notch up 18 years at the BBC, and built up good contacts with some of the Conservative Party's more gregarious politicians. ‘He's a lad's lad,' says one Tory insider. ‘People such as [shadow home secretary] David Davis love him because they can go out and have a proper drink with him.'

It has become common knowledge in political circles that Harri was approached last year about becoming the party's director of communications. According to Tory sources, Harri first spoke to strategy director Steve Hilton and then went to Cameron's Oxfordshire home to discuss the issue. Harri admits: ‘I did have conversations with certain people about possibly taking some role with David Cameron.'

In the end, however, that job went to former News of the World editor Andy Coulson. Some months later Harri exited the BBC to move into the world of agency PR with Fleishman-Hillard, where one of his first roles was advising Zimbabwe's main opp­osition party, the Movement for Democratic Change.

His spell at Fleishman proved to be over in a matter of weeks but former colleagues have fond memories of working with him - even if it was brief.

Gavin Megaw, the agency's head of corporate communications and issues management, says: ‘Guto's an affable guy to have around any office. He stands out because he's likeable but he also has a serious app­roach to getting things done.'

Harri says it was tough to leave Fleishman in the lurch, adding that his former colleagues are ‘brilliant at what they do and wonderful to spend time with'. But he says the prospect of helping Johnson to deliver his vision for London was just ‘too fascinating and inspiring to turn down'.

Two weeks in, the new job seems to be living up to expectations. ‘As Boris predicted when he asked me to take the job, it's a rollercoaster,' Harri enthuses. ‘I'm waking up at 5am full of ideas, but getting home late at night feeling that the to do list is even lon­ger than it was at the start of the day.

There have been the distressing, tragic events like the recent deaths of young people in London that highlight what a crucial and sober task we all have before us at City Hall. But there's a real sense of mission here, there's a real sense that things can be achieved and there's a real sense of fun.'

TURNING POINTSWho was your most notable mentor?Kevin Marsh, when he was editor of the BBC's World At One, gave focus to my political journalism and was a real inspiration. Meanwhile, [Radio Five chief political correspondent] John Pienaar took me under his wing and mentored me on the social demands of being a journalist.

What are the most memorable moments of your career?Being on Tony Blair's plane to Tokyo when he was told that David Kelly had killed himself. Being in Northern Iraq with the Royal Marines as a 23-year-old when there was a sudden stand-off with the Republican guard at the end of the first Gulf war. There were wonderful things when I was the BBC's Rome correspondent for seven months but perhaps the most memorable was joining A-list Italian celebrities such as Giorgio Armani and Sophia Loren in the crowd for the reopening of La Scala in Milan.

What are the qualities you'd like to see from fellow PROs working for the mayor at City Hall?What has been a pleasant surprise on taking this job is to discover there's a highly motivated team of people who seem eager to embrace the new administration and, like me, relish the prospect of working for a mayor who has energy and vision, but also a sense of humour.